The development of high energy battery systems requires the compatibility of electrolytes possessing desirable electrochemical properties with highly reactive anode materials, such as lithium, sodium and the like, and the efficient use of high energy density cathode materials, such as FeS.sub.2, and the like. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has, therefore, been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, to turn to the investigation of nonaqueous electrolyte systems and more particularly to nonaqueous organic electrolyte systems.
A multitude of solutes is known and recommended for use but the selection of a suitable solvent has been particularly troublesome since many of those solvents which are used to prepare electrolytes sufficiently conductive to permit effective ion migration through the solution are reactive with the highly reactive anodes described above. Most investigators in this area, in search of suitable solvents, have concentrated on aliphatic and aromatic nitrogen- and oxygen-containing compounds with some attention given to organic sulfur-, phosphorus- and arsenic-containing compounds. The results of this search have not been entirely satisfactory since many of the solvents investigated still could not be used effectively with extremely high energy density cathode materials and were sufficiently corrosive to lithium anodes to prevent efficient performance over any length of time.
U.S. Pat. No. 3,996,069 discloses a nonaqueous cell utilizing a highly active metal anode, a solid cathode selected from the group consisting of FeS.sub.2, Co.sub.3 O.sub.4, V.sub.2 O.sub.5, Pb.sub.3 O.sub.4, In.sub.2 S.sub.3, and CoS.sub.2, and a liquid organic electrolyte based on 3-methyl-2-oxazolidone in conjunction with a low viscosity cosolvent and a selected solute.
U.S. Pat. No. 4,450,214 discloses a lithium nonaqueous cell employing an FeS.sub.2 -containing or MnO.sub.2 -containing cathode and an organic electrolyte and wherein a lithium halide, such as LiCl, is added to the cell to improve the voltage level on pulse discharge of the cell particularly at low temperatures and/or to stabilize the open circuit voltage.
U.S. Pat. No. 4,416,960 discloses an improved electrolyte for a Li/TiS.sub.2 current producing system, such as a battery, featuring at least one lithium solute of the general formula LiXF.sub.a, where X is selected from a group consisting of B, P, Sb, and As, and wherein a is 4 for B and 6 for P, Sb and As. The solute(s) are supported in a solvent consisting of at least 1,3-dioxolane, and more particularly in a mixture of 1,3-dioxolane (DIOX) and 1,2 dimethoxyethane (DME) from 100/0 to 40/60 by weight.
U.S. Pat. No. 4,129,691 discloses electrolytes for use in lithium primary cells made from a mixture of three organic solvents and an alkaline solute. The first solvent is chosen to have a dielectric constant greater than 35 (e.g., propylene carbonate), the second solvent is a linear polyether with its ether functional groups in the .alpha. position (e.g., 1-2-dimethoxyethane) and the third solvent has a high solvation power for dissolving large quantities of the alkaline salt (e.g. 1-3-dioxolane). The solvents are so chosen that the conductivity maxima of the electrolytes that would be obtained by mixing the solvents in pairs are higher than the conductivity maxima of the electrolytes that would be obtained by using each of the solvents on its own. The positive electrode cell using the electrolyte may be cupric oxide or ferrous disulfide.
While the theoretical energy, i.e., the electrical energy potentially available from a selected anode-cathode couple is relatively easy to calculate, there is a need to choose a nonaqueous electrolyte for such a couple that permits the actual energy produced by an assembled battery to approach the theoretical energy. The problem usually encountered is that it is practically impossible to predict in advance how well, if at all, a nonaqueous electrolyte will function with a selected couple. Thus a cell must be considered as a unit having three parts--a cathode, an anode and an electrolyte--and it is to be understood that the parts of one cell are not predictably interchangeable with parts of another cell. This problem is particularly true of rechargeable cells in which the cells can be discharged and then charged so that the usable cell life can be extended for many charging cycles.
It is an object of the present invention to provide a Li/FeS.sub.2 cell with a specific electrolyte composed of three solvents along with a lithium-based solute that will enable the cell to be a rechargeable cell.
Another object of the present invention is to provide a rechargeable Li/FeS.sub.2 cell with an electrolyte composed of a mixture of 1,3-dioxolane, 1,2-dimethoxyethane and 3-methyl-2-oxazolidone with a solute such as LiI or LiCF.sub.3 SO.sub.3.
It is another object of the present invention to provide an electrolyte for a rechargeable Li/FeS.sub.2 cell in which the electrolyte solvent comprises at least 27 to 33 percent by volume 3-methyl-2-oxazolidone and a ratio volume between 0.8 to 2.2 and 1.1 to 1.9 of 1,3-dioxolane to 1,2-dimethoxyethane mixed with a solute of LiI or LiCF.sub.3 SO.sub.3.
It is another object of the present invention to provide a cost effective rechargeable Li/FeS.sub.2 cell that can be charged and discharged for many cycles.